The present invention relates to a method for measuring the viscosity of a thin-layer material deposited on a substrate.
The invention especially relates to the field of integrated circuits. During manufacturing of integrated circuits, determined materials have to be deposited in very thin layers and the shape of these thin layers has to be modified by appropriate thermal processes that cause the material to flow. The flowing of the material corresponds to a mass transfer resulting from superficial forces, this mass transfer being possible only if the material has a determined viscosity.
Generally, the materials that are deposited in thin layer are not intended to be heated at high temperatures for a sufficiently long time to provide a very substantial flowing. It is simply tried to change, by a determined flowing, the shapes of the patterns of the deposited material, particularly to round up angles. During manufacturing of integrated circuits, a thermal process aiming at achieving a determined flowing of a given material can be properly carried out only if the viscosity of the material, at the selected temperature, is relatively well known. Particularly, the knowledge of this viscosity can serve as a parameter introduced in modelling systems which are constituted by softwares enabling to foresee the behavior and arrangement of microstructures, without involving a high number of actual experiments. Also, it may be advantageous to very accurately know the viscosity of a determined material, at a given temperature, for any other reason than that mentioned above.
The structure and chemical compositions of materials in thin layers do not strictly correspond to that of the corresponding bulk material. Hence, it is not possible to deduce, by conventionally measuring a bulk viscosity, the viscosity of the same material deposited in thin layer.
It is reminded that the measurement of viscosity according to conventional methods consists in disposing a determined amount of material in a tank provided with an aperture and measuring the flowing rate of this material through the aperture.
U.S. Pat. No. 4,813,781 teaches that it is possible to measure the degree of flowing of a material deposited in thin layer by forming on a substrate an array of parallel strips of the material, so that these parallel strips constitute a diffraction grating, by subjecting the grating to the conditions wherein the material flows, by lighting the grating with a monochromatic light beam and by observing the evolution of the light diffracted during flowing. Particularly, the intensity of at least one light spot of the diffracted pattern is measured and a determined distortion level of the pattern strips is deduced therefrom and, hence, a determined flowing value.
This method enables comparing a flowing level of a material to an equal flowing level of the same material previously obtained and which was satisfactory. However, this method does not enable to quantitatively measure the displacements of materials and hence does not enable to determine, for given conditions, the viscosity index of a material.